The field relates to two-stroke cycle, opposed-piston engines. Particularly, the field concerns an air handling system that delivers air to, and transports exhaust from, the inline cylinders of an opposed-piston engine.
A two-stroke cycle engine is an internal combustion engine that completes a cycle of operation with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. The strokes are typically denoted as compression and power strokes. One example of a two-stroke cycle engine is an opposed-piston engine in which two pistons are disposed in the bore of a cylinder for reciprocating movement in opposing directions along the central axis of the cylinder. Each piston moves between a bottom center (BC) location where it is nearest one end of the cylinder and a top center (TC) location where it is furthest from the one end. The cylinder has ports formed in the cylinder sidewall near respective BC piston locations. Each of the opposed pistons controls one of the ports, opening the port as it moves to its BC location, and closing the port as it moves from BC toward its TC location. One of the ports serves to admit charge air into the bore, the other provides passage for the products of combustion out of the bore; these are respectively termed “intake” and “exhaust” ports (in some descriptions, intake ports are referred to as “air” ports or “scavange” ports). In a uniflow-scavenged opposed-piston engine, pressurized charge air enters a cylinder through its intake port as exhaust gas flows out of its exhaust port, thus gas flows through the cylinder in a single direction (“uniflow”)—from intake port to exhaust port.
Charge air and exhaust products flow through the cylinder via an air handling system (also called a “gas exchange” system). Fuel is delivered by injection from a fuel delivery system. As the engine cycles, a control mechanization governs combustion by operating the air handling and fuel delivery systems in response to engine operating conditions. The air handling system may be equipped with an exhaust gas recirculation (“EGR”) system to reduce production of undesirable compounds during combustion.
In an opposed-piston engine, the air handling system moves fresh air into and transports combustion gases (exhaust) out of the engine, which requires pumping work. The pumping work may be done by a gas-turbine driven pump, such as a compressor, and/or by a mechanically-driven pump, such as a supercharger. In some instances, the compressor unit of a turbocharger may feed the inlet of a downstream supercharger in a two-stage pumping configuration. The pumping arrangement (single stage, two-stage, or otherwise) drives the scavenging process, which is critical to ensuring effective combustion, increasing the engine's indicated thermal efficiency, and extending the lives of engine components such as pistons, rings, and cylinders. The pumping work also drives an exhaust gas recirculation system.
FIG. 1 illustrates a turbocharged, two-stroke cycle, opposed-piston engine 10 with uniflow scavenging. The engine 10 has at least one ported cylinder 50. For example, the engine may have one ported cylinder, two ported cylinders, or three or more ported cylinders. Each ported cylinder 50 has a bore 52 and longitudinally-spaced intake and exhaust ports 54 and 56 formed or machined near respective ends of a cylinder wall. Each of the intake and exhaust ports includes one or more circumferential arrays of openings or perforations. In some descriptions, each opening is referred to as a “port”; however, the construction of one or more circumferential arrays of such “ports” is no different than the port constructions shown in FIG. 1. Pistons 60 and 62 are slidably disposed in the bore 52 with their end surfaces 61 and 63 in opposition. The piston 60 controls the intake port 54, and the piston 62 controls the exhaust port 56. In the example shown, the engine 10 further includes at least one crankshaft; preferably, the engine includes two crankshafts 71 and 72. The intake pistons 60 of the engine are coupled to the crankshaft 71, and the exhaust pistons 62 to the crankshaft 72.
As the pistons 60 and 62 near their TC locations, a combustion chamber is defined in the bore 52 between the end surfaces 61 and 63 of the pistons. Combustion timing is frequently referenced to the point in the compression cycle where minimum combustion chamber volume occurs because the pistons end surfaces are nearest each other; this point is referred to as “minimum volume.” Fuel is injected directly into cylinder space located between the end surfaces 61 and 63. In some instances injection occurs at or near minimum volume; in other instances, injection may occur before minimum volume. Fuel is injected through one or more fuel injector nozzles positioned in respective openings through the sidewall of the cylinder 50. Two such nozzles 70 are shown. The fuel mixes with charge air admitted into the bore 52 through the intake port 54. As the air-fuel mixture is compressed between the end surfaces 61 and 63, the compressed air reaches a temperature and a pressure that cause the fuel to ignite. Combustion follows.
With further reference to FIG. 1, the engine 10 includes an air handling system 80 that manages the transport of charge air to, and exhaust gas from, the engine 10. A representative air handling system construction includes a charge air subsystem and an exhaust subsystem. In the air handling system 80, a charge air source receives intake air and processes it into pressurized air (hereinafter “charge air”). The charge air subsystem transports the charge air to the intake ports of the engine. The exhaust subsystem transports exhaust products from exhaust ports of the engine for delivery to other exhaust components.
The air handling system 80 may include a turbocharger 120 with a turbine 121 and a compressor 122 that rotate on a common shaft 123. The turbine 121 is in fluid communication with the exhaust subsystem and the compressor 122 is in fluid communication with the charge air subsystem. The turbocharger 120 extracts energy from exhaust gas that exits the exhaust ports 56 and flows into an exhaust channel 124 directly from the exhaust ports 56, or from an exhaust manifold assembly 125 that collects exhaust gasses output through the exhaust ports 56. In this regard, the turbine 121 is rotated by exhaust gas passing through it to an exhaust outlet channel 128. This rotates the compressor 122, causing it to generate charge air by compressing fresh air. The charge air subsystem may include a supercharger 110 and an intake manifold 130. The charge air subsystem may further include at least one charge air cooler (hereinafter, “cooler”) to receive and cool the charge air before delivery to the intake port or ports of the engine. The charge air output by the compressor 122 flows through a charge air channel 126 to a cooler 127, whence it is pumped by the supercharger 110 to the intake ports. Charge air compressed by the supercharger 110 is output to an intake manifold 130. The intake ports 54 receive charge air pumped by the supercharger 110, through the intake manifold 130. A second cooler 129 may be provided between the outlet of the supercharger 110 and the inlet of the intake manifold 130.
In some aspects, the air handling system 80 may be constructed to reduce undesirable emissions produced by combustion by recirculating a portion of the exhaust gas produced by combustion through the ported cylinders of the engine. The recirculated exhaust gas is mixed with charge air to lower peak combustion temperatures, which reduces production of the undesirable emissions. This process is referred to as exhaust gas recirculation (“EGR”). The EGR construction shown obtains a portion of the exhaust gasses flowing from the port 56 during scavenging and transports them via an EGR channel 131 external to the cylinders into the incoming stream of inlet air in the charge air subsystem. The recirculated exhaust gas flows through the EGR channel 131 under the control of a valve 138 (referred to as the “EGR valve”).
FIG. 2 shows the air handling system 80 of FIG. 1 in schematic detail. In this regard, the charge air subsystem provides intake air to the compressor 122. As the compressor 122 rotates, compressed air flows from the compressor's outlet, through the charge air channel 126, and into the supercharger 110. Charge air pumped by the supercharger 110 flows through the cooler 129 into the intake manifold 130. Pressurized charge air is delivered from the intake manifold 130 to the intake ports of the cylinders 50, which are supported in a cylinder block 160. In some aspects, the engine may include a recirculation channel 112 that couples the outlet of the supercharger 110 to its inlet. Provision of a valve 139 in the recirculation channel 112 allows the charge air flow to the cylinders to be varied by modulation of charge air pressure downstream of the supercharger outlet.
Exhaust gasses from the exhaust ports of the cylinders 50 flow from the exhaust manifold 125 into the turbine 121, and from the turbine into the exhaust outlet channel 128. In some instances, one or more after-treatment devices (AT) 162 are provided in the exhaust outlet channel 128. Exhaust is recirculated through the EGR channel 131, under control of the EGR valve 138. The EGR channel 131 is in fluid communication with the charge air subsystem via an EGR mixer (not shown).
Opposed-piston engines have included various constructions designed to transport engine gasses (charge air, exhaust) into and out of the cylinders. For example, U.S. Pat. No. 1,517,634 describes an early opposed-piston aircraft engine that made use of a multi-pipe exhaust manifold having a pipe in communication with the exhaust area of each cylinder that merged with the pipes of the other cylinders into one exhaust pipe. The manifold was mounted to one side of the engine.
Later, in the 1930s, the Jumo 205 family of opposed-piston aircraft engines established a basic air handling architecture for dual-crankshaft, inline, opposed-piston engines. Each engine was equipped with multi-pipe exhaust manifolds that bolted to opposite sides of an inline cylinder block with six cylinders so as to place a respective pair of opposing pipes in communication with the annular exhaust area of each cylinder. The outlet pipe of each exhaust manifold was connected to a respective one of two entries to a turbine. A two-stage pressure charging system provided pressurized charge air. The pressurized charge air output by the second stage flowed through an intercooler that straddled the bottom of the engine. The charge air then flowed out from the intercooler through pipes to intake conduits which ran along the sides of the engine like the exhaust manifolds. The constructions of the exhaust and intake systems imposed considerable burdens that resulted in increased engine volume, weight and cost, and reduced performance.
The prior art exhaust manifolds extracted a penalty in increased engine size and weight. Each individual pipe required structural support in order to closely couple the pipe opening with the annular exhaust space of a cylinder. Typically, the support was in the form of a flange at the end of each pipe with an area sufficient to receive threaded fasteners for sealably fastening the flange to a corresponding area on a side of the cylinder block. The flanges of each manifold were arranged row-wise in order to match the inline arrangement of the cylinders. The flange width restricted cylinder-to-cylinder spacing, which required the engine to be comparatively heavy and large.
The prior art intake construction for the Jumo 205 required an intercooler mounted to the engine that was coupled to two intake conduits, one on each side of the engine, via pipes and fittings that introduced length, bends, and constrictions into the charge air pathway between the cooler and the conduits. The variations in direction and flow resistance resulted in parasitic eddies and oscillations that produced sharp variations in charge air pressure from cylinder to cylinder and that changed in response to changing engine conditions. Surges, spikes, and other sharp inconsistencies and asymmetries in the pressure of charge air delivered to the intake ports can produce inconsistent combustion and incomplete scavenging, making the engine less efficient, dirtier-running, and more difficult to control over the range of engine operating conditions that the engine was designed for.
The Jumo intake construction also included a manifold structure formed inside the cylinder block by subdividing space into individual compartments for the inlet areas of the cylinders. Each compartment opened through opposing sides of the cylinder block to receive charge air from the intake conduits. Such a manifold structure may produce charge air pressure differentials between inlet ports, which can cause variations in combustion and scavenging as engine operating conditions change.